SYDNEY (Reuters) - Rising acidity in the ocean caused by
seas absorbing greenhouse carbon dioxide could make low-lying
island nations like Kiribati and the Maldives more vulnerable
to storms as their coral reefs struggle to survive, say
scientists.

Carbon dioxide in the atmosphere is at its highest level in
the past 650,000 years, possible 23 million years, and half has
now been dissolved into the oceans making them more acidic.

Ocean acidification, which is projected to spread
extensively north from the Antarctic by 2100, makes it
difficult or impossible for some animals, like coral and
starfish, to produce their shells and skeletons.

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"If ocean acidification weakens the structure of
reef-forming corals and algae, tropical systems (islands) will
be more vulnerable to physical impacts from storms and
cyclones," said a new report by some of the world's leading
marine scientists.

"By 2100, it is expected that some reefs will become
marginal and reef calcification will decline," said the report,
by the Antarctic Climate & Ecosystems Cooperative Research
Centre, released on Monday.

The report cited Kiribati in the South Pacific and the
Maldives in the Indian Ocean as being more vulnerable to
tropical storms if ocean acidification continues to rise.

"These impacts will also directly affect important
commercial, recreational or subsistence reef fisheries where
the target species depend on reef habitats," said the report,
released at an ocean acidification conference in Hobart.

Ocean acidification is when carbon dioxide dissolves in the
sea forming a weak acid, carbonic acid. Human-induced carbon
dioxide has largely been produced by burning fossil fuels,
agricultural practices and concrete production.

"The ocean is a major sink for CO2 emissions and has
absorbed about 48 percent of the CO2 emitted by human
activities since preindustrial times," said the report.

FOOD CHAIN THREATENED

Ocean acidification is already affecting the cold water
marine life of the Southern Ocean where most carbon dioxide has
dissolved and U.S. researchers said it was now appearing on the
Pacific North American continental shelf.

"The Southern Ocean is a biogeochemical 'harbinger' for the
impacts of acidification that will spread throughout the global
ocean," said the report.

By 2060, Antarctic polar waters would experience carbonate
ion concentrations so low that one form of calcium carbonate,
aragonite, will not be available for organisms to build shells.

Ocean acidification may also interfere with the respiration
of fish, the larval development of marine organisms and the
ability of oceans to absorb nutrients and toxins.

"Ocean acidification is likely to have an ecological
cascade effect right up to parts of the food web that are
important to human beings, such as fish and shell fish," said
research scientist Will Howard from the Antarctic research
centre.

The report said ice cores showed that the current rate of
increase of carbon dioxide in the atmosphere is 100 times
greater than the most rapid increases experienced in the last
650,000 years. Sedimentary records suggest carbon dioxide
levels were higher than at anytime in the last 23 million
years.

It said atmospheric carbon dioxide levels are expected to
reach about double pre-industrial levels within this century,
resulting in an acidification of oceans three times the level
experienced during the last major rise in carbon dioxide during
the last glacial period 15,000 years ago.

"Many (marine) species have taken millennia to evolve and
it is unknown whether they can (or will) be able to adapt to
the relatively rapid rate of ocean acidification, in the order
of decades not millennia," said the report.